At present, none. The only route which has both Southern and Southeastern trains on it is BML:- which of course also has the GX 387
Technically Southeastern High Speed has a thing where there’s a small chance of a 377 subbing in on a Southeastern train (which of course is typically using the 375). I’ve seen it before at Ashford!
Southern and Southeastern have very little overlap. Even the one on BML is "forced", as in the SE runs were added to fill in some gaps and are "diverted services" rather than actual. The only place you'll see 375s and 377s running "together" is between London Bridge and Millwall before the Kent and Sussex lines split off and the greens go one way, blues go the other
There's a 377 in southeastern livery that runs on the sehs in rl but it goes to victoria. No 375s operate through dartford. On tsw you will see 375s on parts of the line on the London to Brighton route and there's a short service you can drive between east Croydon and Redhill.
Southeastern definitely runs in and out of Victoria. Not as often as in game, and not as often as from London Bridge or St Pancras, but they do have Victoria services
Southeastern run up from Victoria, past Grosvenor sidings, over the bridge then all turn left, no southern services turn left, they all go straight on, so really they don't share lines at all. Even Victoria itself is split into Southern and Southeastern sides for the most part, with P1-P7 being SE, P8 being shared (kinda) and 9 - 15 being GTR I guess you could say the same for southern, with all services from the sussex lines terminating at LBG I believe